How to alter columns without losing data?

[feniljariwala82]

I followed the guide provided in the documentation. My use case is to make a column foreign key. I used alterTable method and added my changes to the column. I ran the migrations, it didn't worked. But when I added alter() method to the columns, it worked! I thought alterTable will alter columns too. Can you please guide me what happened here?

[tomgobich]

My understanding is alterTable, by default, can handle adding and removing columns (since removing has its own method(s)). The alter method comes into play when you need to actually alter an existing column.

So, for example, if I need to add a new column "summary" and alter an existing column called "body" from type varchar to text, my migration would look like this:

this.schema.alterTable(this.tableName, table => {
  table.string('summary') // 👈 adds new column
  table.text('body').alter() // 👈 changes type from varchar to text
})

Here's the documentation on alter() if you're interested:
https://knexjs.org/#Schema-alter